Cooling electronic devices in a data center

ABSTRACT

A thermosiphon includes a condenser; an evaporator that includes a fluid channel and a heat transfer surface, the heat transfer surface defining a plurality of fluid pathways in the fluid channel that extend through the fluid channel, the evaporator configured to thermally couple to one or more heat-generating electronic devices; and a transport member that fluidly couples the condenser and the evaporator, the transport member including a liquid conduit that extends through the transport member to deliver a liquid phase of a working fluid into the fluid pathways, the transport member further including a surface to vertically enclose the plurality of fluid pathways.

TECHNICAL FIELD

This document relates to systems and methods for providing cooling toelectronic equipment, such as computer server racks and relatedequipment in computer data centers, with a thermosiphon.

BACKGROUND

Computer users often focus on the speed of computer microprocessors(e.g., megahertz and gigahertz). Many forget that this speed often comeswith a cost—higher power consumption. This power consumption alsogenerates heat. That is because, by simple laws of physics, all thepower has to go somewhere, and that somewhere is, in the end, conversioninto heat. A pair of microprocessors mounted on a single motherboard candraw hundreds of watts or more of power. Multiply that figure by severalthousand (or tens of thousands) to account for the many computers in alarge data center, and one can readily appreciate the amount of heatthat can be generated. The effects of power consumed by the criticalload in the data center are often compounded when one incorporates allof the ancillary equipment required to support the critical load.

Many techniques may be used to cool electronic devices (e.g.,processors, memories, networking devices, and other heat generatingdevices) that are located on a server or network rack tray. Forinstance, forced convection may be created by providing a coolingairflow over the devices. Fans located near the devices, fans located incomputer server rooms, and/or fans located in ductwork in fluidcommunication with the air surrounding the electronic devices, may forcethe cooling airflow over the tray containing the devices. In someinstances, one or more components or devices on a server tray may belocated in a difficult-to-cool area of the tray; for example, an areawhere forced convection is not particularly effective or not available.

The consequence of inadequate and/or insufficient cooling may be thefailure of one or more electronic devices on the tray due to atemperature of the device exceeding a maximum rated temperature. Whilecertain redundancies may be built into a computer data center, a serverrack, and even individual trays, the failure of devices due tooverheating can come at a great cost in terms of speed, efficiency, andexpense.

Thermosiphons are heat exchangers that operate using a fluid thatundergoes a phase change. A liquid form of the fluid is vaporized in anevaporator, and heat is carried by the vapor form of the fluid from theevaporator to a condenser. In the condenser, the vapor condenses, andthe liquid form of the fluid is then returned via gravity to theevaporator. Thus, the fluid circulates between the evaporator and thecondenser without need of a mechanical pump.

SUMMARY

This disclosure describes a thermosiphon system that may be used to coolone or more heat generating devices mounted in a rack of a computer datacenter (e.g., on a motherboard, server board, or otherwise). Thethermosiphon system, is some example implementations, includes acondenser and a flow boiling evaporator that are fluidly coupledtogether with a conduit. In some aspects, the flow boiling evaporatorincludes one or more fluid paths through which a liquid phase of aworking fluid flows as it receives heat from the one or more heatgenerating devices and changes to a vapor or mixed (liquid and vapor)phase.

In an example general implementation, a thermosiphon includes acondenser; an evaporator that includes a fluid channel and a heattransfer surface, the heat transfer surface defining a plurality offluid pathways in the fluid channel that extend through the fluidchannel, the evaporator configured to thermally couple to one or moreheat-generating electronic devices; and a transport member that fluidlycouples the condenser and the evaporator, the transport member includinga liquid conduit that extends through the transport member to deliver aliquid phase of a working fluid into the fluid pathways, the transportmember further including a surface to vertically enclose the pluralityof fluid pathways.

In a first aspect combinable with the general implementation, the fluidchannel is oriented transverse to the fluid pathways.

In a second aspect combinable with any of the previous aspects, thetransport member further includes a vapor conduit that extends throughthe transport member to receive a mixed-phase of the working fluid fromthe fluid pathways.

In a third aspect combinable with any of the previous aspects, the fluidpathways are configured to transfer heat from the heat-generatingelectronic devices to the working fluid to change the working fluid fromthe liquid phase to the mixed-phase between inlets of the fluid pathwaysand outlets of the fluid pathways.

In a fourth aspect combinable with any of the previous aspects, theinlets of the fluid pathways are positioned at a first end of the fluidpathways and the outlets of the fluid pathways are positioned at asecond end of the fluid pathways opposite the first end.

In a fifth aspect combinable with any of the previous aspects, theinlets of the fluid pathways are positioned at a midpoint of the fluidpathways, and the outlets of the fluid pathways are positioned atopposed ends of the fluid pathways.

In a sixth aspect combinable with any of the previous aspects, the heattransfer surface includes a plurality of fins or ridges, and theplurality of fins or ridges form the plurality of fluid pathways in thefluid channel, and the plurality of fluid pathways extend transverse tothe fluid channel.

In a seventh aspect combinable with any of the previous aspects, theheat transfer surface includes a plurality of fins or ridges, and theplurality of fins or ridges form the plurality of fluid pathways in thefluid channel, and the plurality of fluid pathways extend parallel withthe fluid channel.

In an eighth aspect combinable with any of the previous aspects, thetransport member further includes a heat transfer interface between theliquid conduit and the vapor conduit to transfer heat from the workingfluid in the vapor conduit to the working fluid in the liquid conduit.

In a ninth aspect combinable with any of the previous aspects, theliquid phase is at or near a saturation temperature of the workingfluid.

In a tenth aspect combinable with any of the previous aspects, the vaporconduit includes at least two vapor conduits.

In an eleventh aspect combinable with any of the previous aspects, atleast a portion of the vapor conduit is positioned in an upper half ofthe transport member, and at least a portion of the liquid conduit ispositioned in a lower half of the transport member.

In a twelfth aspect combinable with any of the previous aspects, thetransport member includes a condenser end that includes an inlet of theliquid conduit that is offset in the transport member relative to anoutlet of the vapor conduit in the condenser end of the transportmember.

In a thirteenth aspect combinable with any of the previous aspects, across sectional area of the liquid conduit and a cross-sectional area ofthe vapor conduit is based, at least in part, on a heat load of theheat-generating electronic devices.

In a fourteenth aspect combinable with any of the previous aspects, thetransport member slopes from the condenser to the evaporator, and amagnitude of the slope defines, at least in part, a liquid head of theworking fluid.

In a fifteenth aspect combinable with any of the previous aspects, theliquid head of the working fluid is equal to a sum of a plurality ofpressure losses in a closed loop fluid circuit that includes the liquidconduit, the evaporator, the vapor conduit, and the condenser.

A sixteenth aspect combinable with any of the previous aspects furtherincludes a spacer positioned in the fluid channel.

In another example general implementation, a method for cooling heatgenerating electronic devices in a data center includes flowing a liquidphase of a working fluid from a condenser of a thermosiphon to anevaporator of the thermosiphon in a transport member of thethermosiphon; flowing the liquid phase of the working fluid into a fluidchannel of the evaporator from the transport member; flowing the liquidphase of the working fluid from the fluid channel to a plurality offluid pathways that extend through the fluid channel and are enclosedbetween the evaporator and the transport member; boiling at least aportion of the liquid working fluid flowing in the plurality of fluidpathways based on a transfer of heat from at least one data center heatgenerating device that is thermally coupled to the evaporator; andflowing a mixed phase of the working fluid out of the plurality of fluidpathways into at least one vapor conduit of the transport member and tothe condenser.

A first aspect combinable with the general implementation furtherincludes transferring heat from the heat-generating electronic devicesto the working fluid through a heat transfer surface that forms theplurality of fluid pathways; and changing the working fluid from theliquid phase to the mixed-phase between inlets of the fluid pathways andoutlets of the fluid pathways.

A second aspect combinable with any of the previous aspects furtherincludes flowing the liquid phase into the inlets of the fluid pathwayspositioned at a first end of the fluid pathways; and flowing the mixedphase out of the outlets of the fluid pathways positioned at a secondend of the fluid pathways opposite the first end.

A third aspect combinable with any of the previous aspects furtherincludes flowing the liquid phase into the inlets of the fluid pathwayspositioned at a midpoint of the fluid pathways; and flowing the mixedphase out of the outlets of the fluid pathways positioned at opposedends of the fluid pathways.

A fourth aspect combinable with any of the previous aspects furtherincludes flowing the working fluid through a plurality of fins or ridgespositioned that form the plurality of fluid pathways in the fluidchannel.

In a fifth aspect combinable with any of the previous aspects, flowingthe working fluid through a plurality of fins or ridges that form theplurality of fluid pathways in the fluid channel includes flowing theworking fluid transversely from the fluid channel through the pluralityof fluid pathways.

A sixth aspect combinable with any of the previous aspects furtherincludes transferring heat from the working fluid in the vapor conduitto the working fluid in the liquid conduit through a heat transferinterface between the liquid conduit and the vapor conduit in thetransport member.

In a seventh aspect combinable with any of the previous aspects, theliquid phase is at or near a saturation temperature of the workingfluid.

An eighth aspect combinable with any of the previous aspects furtherincludes flowing the mixed phase of the working fluid through the atleast one vapor conduit to the condenser.

In another example general implementation, a data center cooling systemincludes a tray sub-assembly configured to engage with a rack; a supportboard mounted on the tray sub-assembly, the support board including aheat-generating computing device; and a thermosiphon system. Thethermosiphon system includes a condenser; a flow boiling evaporator thatincludes at least one fluid pathway configured to receive a flow of aworking fluid in liquid phase and output the flow of the working fluidin a mixed liquid-vapor phase based on heat transferred from theheat-generating computing device to the flow of the working fluid; and atransport tubular that fluidly couples the condenser and the evaporator.

In a first aspect combinable with the general implementation, theevaporator includes a fluid channel and a plurality of fluid pathwaysthat extend through the fluid channel.

In a second aspect combinable with any of the previous aspects, thetransport tubular includes a liquid carrier that extends through thetransport tubular and is oriented transverse to the fluid pathways todeliver the flow of the working fluid in liquid phase into the fluidpathways; and a vapor carrier that extends through the transport tubularto receive the flow of the working fluid in mixed liquid-vapor phasefrom the fluid pathways.

In a third aspect combinable with any of the previous aspects, theplurality of fluid pathways are defined between a plurality of heattransfer surfaces.

In a fourth aspect combinable with any of the previous aspects, inletsof the plurality fluid pathways are positioned at a first end of thefluid pathways and outlets of the plurality of fluid pathways arepositioned at a second end of the fluid pathways opposite the first end.

In a fifth aspect combinable with any of the previous aspects, inlets ofthe plurality of fluid pathways are positioned near or at a midpoint ofthe fluid pathways, and outlets of the plurality of fluid pathways arepositioned at opposed ends of the fluid pathways.

Various implementations of a data center cooling system according to thepresent disclosure may include one, some, or all of the followingfeatures. For example, a thermosiphon assembly of the data centercooling system may provide for increased thermal performance by removingmore heat from heat-generating electronic devices as compared toconventional, pool boiling thermosiphons. As another example, thethermosiphon assembly may better match a particular thermal load of aheat-generating electronic device, thereby providing a more accurateamount of cooling without being over or undersized. As yet anotherexample, the thermosiphon assembly may be flow balanced as compared toconventional thermosiphons, thereby more efficiently circulating aworking fluid through the assembly.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a front view of a server rack and a server-racksub-assembly configured to mount within a rack used in a data centerenvironment.

FIGS. 2A-2B illustrate schematic side and top views, respectively, of aserver rack sub-assembly that includes an example implementation of athermosiphon cooling system.

FIGS. 3A-3C illustrate schematic cross-sectional end, isometric, and topviews, respectively, of an evaporator of an example implementation of athermosiphon cooling system.

FIGS. 4A-4C illustrate schematic cross-sectional end, isometric, andtop, respectively, of an evaporator of another example implementation ofa thermosiphon cooling system.

FIGS. 5A-5C illustrate schematic side and isometric views of a condenserof example implementations of a thermosiphon cooling system.

FIGS. 6A-6B illustrate schematic side and top cross-sectional views,respectively, of an evaporator of another example implementation of athermosiphon cooling system.

FIG. 6C illustrates a schematic top cross-sectional view of anevaporator of another example implementation of a thermosiphon coolingsystem.

DETAILED DESCRIPTION

FIG. 1 illustrates an example system 100 that includes a server rack105, e.g., a 13 inch or 19 inch server rack, and multiple server racksub-assemblies 110 mounted within the rack 105. Although a single serverrack 105 is illustrated, server rack 105 may be one of a number ofserver racks within the system 100, which may include a server farm or aco-location facility that contains various rack mounted computersystems. Also, although multiple server rack sub-assemblies 110 areillustrated as mounted within the rack 105, there might be only a singleserver rack sub-assembly. Generally, the server rack 105 definesmultiple slots 107 that are arranged in an orderly and repeating fashionwithin the server rack 105, and each slot 107 is a space in the rackinto which a corresponding server rack sub-assembly 110 can be placedand removed. For example, the server rack sub-assembly can be supportedon rails 112 that project from opposite sides of the rack 105, and whichcan define the position of the slots 107.

The slots 107, and the server rack sub-assemblies 110, can be orientedwith the illustrated horizontal arrangement (with respect to gravity).Alternatively, the slots 107, and the server rack sub-assemblies 110,can be oriented vertically (with respect to gravity), although thiswould require some reconfiguration of the evaporator and condenserstructures described below. Where the slots are oriented horizontally,they may be stacked vertically in the rack 105, and where the slots areoriented vertically, they may be stacked horizontally in the rack 105.

Server rack 105, as part of a larger data center for instance, mayprovide data processing and storage capacity. In operation, a datacenter may be connected to a network, and may receive and respond tovarious requests from the network to retrieve, process, and/or storedata. In operation, for example, the server rack 105 typicallyfacilitates the communication of information over a network with userinterfaces generated by web browser applications of users who requestservices provided by applications running on computers in thedatacenter. For example, the server rack 105 may provide or help providea user who is using a web browser to access web sites on the Internet orthe World Wide Web.

The server rack sub-assembly 110 may be one of a variety of structuresthat can be mounted in a server rack. For example, in someimplementations, the server rack sub-assembly 110 may be a “tray” ortray assembly that can be slidably inserted into the server rack 105.The term “tray” is not limited to any particular arrangement, butinstead applies to motherboard or other relatively flat structuresappurtenant to a motherboard for supporting the motherboard in positionin a rack structure. In some implementations, the server racksub-assembly 110 may be a server chassis, or server container (e.g.,server box). In some implementations, the server rack sub-assembly 110may be a hard drive cage.

Referring to FIGS. 2A-2B, the server rack sub-assembly 110 includes aframe or cage 120, a printed circuit board 122, e.g., motherboard,supported on the frame 120, one or more heat-generating electronicdevices 124, e.g., a processor or memory, mounted on the printed circuitboard 122, and a thermosiphon system 130. One or more fans 126 can alsobe mounted on the frame 120.

The frame 120 can include or simply be a flat structure onto which themotherboard 122 can be placed and mounted, so that the frame 120 can begrasped by technicians for moving the motherboard into place and holdingit in position within the rack 105. For example, the server racksub-assembly 110 may be mounted horizontally in the server rack 105 suchas by sliding the frame 120 into the slot 107 and over a pair of railsin the rack 105 on opposed sides of the server rack sub-assembly110—much like sliding a lunch tray into a cafeteria rack. Although FIGS.2A-2B illustrate the frame 120 extending below the motherboard 122, theframe can have other forms (e.g., by implementing it as a peripheralframe around the motherboard) or may be eliminated so that themotherboard itself is located in, e.g., slidably engages, the rack 105.In addition, although FIG. 2A illustrates the frame 120 as a flat plate,the frame 120 can include one or more side walls that project upwardlyfrom the edges of the flat plate, and the flat plate could be the floorof a closed-top or open-top box or cage.

The illustrated server rack sub-assembly 110 includes a printed circuitboard 122, e.g., a motherboard, on which a variety of components aremounted, including heat-generating electronic devices 124. Although onemotherboard 122 is illustrated as mounted on the frame 120, multiplemotherboards may be mounted on the frame 120, depending on the needs ofthe particular application. In some implementations, the one or morefans 126 can be placed on the frame 120 so that air enters at the frontedge (at the left hand side in FIGS. 2A-2B) of the server racksub-assembly 110, closer to the front of the rack 105 when thesub-assembly 110 is installed in the rack 105, flows (as illustrated)over the motherboard 122, over some of the heat generating components onthe motherboard 122, and is exhausted from the server rack assembly 110at the back edge (at the right hand side), closer to the back of therack 105 when the sub-assembly 110 is installed in the rack 105. The oneor more fans 126 can be secured to the frame 120 by brackets. Thus, thefans 126 can pull air from within the frame 120 area and push the airafter it has been warmed out the rack 105. An underside of themotherboard 122 can be separated from the frame 120 by a gap.

The thermosiphon system 130 includes an evaporator 132, a condenser 134mounted on a base 139, and a transport member 136 connecting theevaporator 132 to the condenser 134. The evaporator 132 contacts theelectronic device 124 so that heat is drawn by conductive heat transferfrom the electronic device 124 to the evaporator 132. For example, theevaporator 132 is in conductive thermal contact with the electronicdevice 124. In particular, the bottom of the evaporator 132 contacts thetop of the electronic device 124. In operation, heat from the electronicdevice 124 causes a working fluid in the evaporator 132 to evaporate.The vapor then passes through transport member 136 to the condenser 134.Heat is radiated away from the condenser 134, e.g., into air around thecondenser 134 or into air blown or drawn by the one or more fans 126that passes across the condenser 134, a heat transfer surface 148 (e.g.,finned surface), or both, causing the working fluid to condense. Asshown in FIG. 2A, the condenser 134 can be located between the one ormore fans 126 from the evaporator 132, but could also be located on anopposite side of one or more of fans 126 (e.g., near an edge of thesub-assembly 110).

As shown in FIG. 2A, the transport member 136 is at a slight (non-zero)angle so that gravity causes the condensed working fluid to flow backthrough the transport member 136 to the evaporator 132. Thus, in someimplementations, at least a portion of the transport member 136 is notparallel to the main surface of the frame 120. For example, thecondenser-side end of the transport member 136 can be about 1-5 mm,e.g., 2 mm, above the evaporator-side end of the transport member 136.However, it is also possible for the transport member 136 to behorizontal tube, or even at a slightly negative angle (although thepositive angle provides an advantage of gravity improving flow of theliquid from the condenser to the evaporator). Because there can bemultiple heat generating electronic devices on a single motherboard,there can be multiple evaporators on the motherboard, where eachevaporator corresponds to a single electronic device. As shown in FIG.2A, there is a first evaporator 132 and a second evaporator 132 as wellas a first electronic device 124 and a second electronic device 124. Thetransport member 136 connecting the first evaporator to the secondevaporator can be level.

During operation, the top surface of the working fluid (as a liquid)inside the condenser 134 will be above the top surface liquid height ofthe working fluid in the evaporator 132, e.g., by 1 to 10 mm. It can beeasier to achieve this with a transport member 136 that is at a slight(positive non-zero) angle, but proper selection of the thermal andmechanical properties of the working fluid in view of the expected heattransport requirements for the thermosiphon system 130 may still achievethis for a transport member 136 that is horizontal or at a slightlynegative angle. During operation, the liquid phase of a working fluidcan flow through a liquid conduit of the transport member 136, and avapor phase (or mixed vapor-liquid phase) of the working fluid can flowthrough a vapor conduit of the transport member 136.

FIGS. 2A-2B illustrate a thermosiphon system 130 with multipleevaporators 132; each evaporator 132 can contact a different electronicdevice 124, or multiple evaporators 132 could contact the sameelectronic device, e.g., if the electronic device is particularly largeor has multiple heat generating regions. The multiple evaporators 132can be connected by the transport member 136 to the condenser 134 inseries, e.g., a single transport member 136 connects the condenser 134to a first evaporator 132 and a second evaporator 132. Alternatively,some or all of the multiple evaporators 132 can be connected by multipletransport members 136 to the condenser 134 in parallel, e.g., a firsttransport member connects the condenser to a first evaporator, and asecond transport member connects the condenser 134 to a secondevaporator. Advantages of a serial implementation may be fewer tubes,whereas an advantage of parallel tubes is that the tube diameters can besmaller.

FIGS. 2A-2B illustrate a thermosiphon system 130 in which a commontransport member 136 is used for both the condensate flow from thecondenser 134 to the evaporator 132 and for vapor (or mixed phase) flowfrom the evaporator 132 to the condenser 134. Thus, in thisimplementation the fluid coupling between the evaporator 132 and thecondenser 134 consists of a combined condensate and vapor transfer line136. A potential advantage of the combined condensate and vapor transferline is that the transport member 136 can be connected to a side of thecondenser, reducing the vertical height of the system relative to asystem with a separate line for the vapor, since the vapor line istypically coupled to or near the top of the evaporator. The transportmember 136 can be a flexible tube or pipe, e.g., of copper or aluminum.

As shown more fully with reference to FIGS. 3A-3C, the thermosiphonsystem 130 includes a flow-boiling evaporator 132 (or evaporators 132)rather than a pool boiling evaporator. In the example flow-boilingevaporator 132, the liquid working fluid that enters the evaporator 132boils as it flows through the evaporator 132 as the liquid absorbs heatfrom electronic device 124 (or devices 124). In comparison, in a poolboiling evaporator, the liquid phase of the working fluid “pools” in theevaporator (e.g., at the lowest elevation) and slowly boils off, withvapor from the liquid phase rising above the pool of liquid workingfluid. As the liquid phase of the working fluid boils, a vapor phase ofthe working fluid is formed, thereby decreasing a density of the workingfluid as it flows in the evaporator. The decreased density of theworking fluid may increase a velocity of the flowing working fluidthrough the evaporator 132, thereby further causing the liquid phase toflow through the evaporator 132. This increased velocity may result inincreased convective heat transfer in the evaporator 132 and,consequently a more efficient heat transfer process between theelectronic devices 124 and the working fluid in the evaporator 132.

In some aspects, a pressure drop through the thermosiphon system 130 isbalanced (e.g., substantially or exactly). A “pumping force” tocirculate a flow of the working fluid through the thermosiphon system130 may be provided by a liquid head (e.g., an amount of liquid phase ofthe working fluid built up in the condenser 134). This liquid headpressure may be equal (e.g., substantially or equally) to a sum of allpressure drops through the system 130. The pressure drops include, forexample, a pressure drop of the liquid phase from the condenser 134 tothe evaporator 132 through the transport member 136, a pressure drop asthe liquid phase flow-boils to the vapor (or mixed) phase in theevaporator 132, and a pressure drop of the vapor (or mixed) phaseflowing through the transport member 136 from the evaporator 132 to thecondenser 134. In some aspects, to ensure proper flow of the workingfluid in a natural flow system (e.g., without mechanical pumping), thetotal pressure drop is equal or substantially equal to the liquid headavailable in the system 130.

In some aspects of the presently described implementations, a vaporconduit that extends through transport member 136 of thermosiphon system130 may be larger (e.g., in cross-sectional area) than a liquid conduitthat extends through the transport member 136. For example, by having alarger (e.g., cross-section) vapor conduit, proper flow of the workingfluid may be assisted or achieved because, for instance, the vapor (ormixed) phase of the working fluid is less dense that the liquid phase.Thus, in order to reduce pressure losses associated with circulating(e.g., naturally) the vapor (or mixed) phase through the vapor conduit,as compared to pressure losses associated with circulating (e.g.,naturally) the liquid phase through the liquid conduit, the vaporconduit flow area may be larger. In some aspects, the vapor conduit flowarea may be a sum of flow areas of two or more vapor conduits thatextend through the transport member 136.

FIGS. 3A-3C illustrate schematic cross-sectional side, isometric, andtop views, respectively, of an evaporator of an example implementationof a thermosiphon cooling system, such as the thermosiphon coolingsystem 130 shown in FIGS. 2A-2B. As illustrated in the sidecross-section of FIG. 3A, an evaporator end 140 of the transport member136 is mounted on the evaporator 132. In this implementation of thetransport member 136, the member 136 includes a single liquid conduit144 and two vapor conduits 146.

As shown, the liquid conduit 144 includes outlet 150 and extends (e.g.,in a “cross” shaped cross-section) through a middle portion of thetransport member 136 and above a heat transfer surface 148 of theevaporator 132. The heat transfer surface 148, in this implementation,is positioned over a fluid channel 154 of the evaporator 132 that isparallel (e.g., exactly or substantially) to the liquid conduit 144.Also, the heat transfer surface 148, in this example, includes multiplefins 159 that form fluid pathways 160 across the evaporator 132 (e.g.,transverse to the liquid conduit 144) within the fluid channel 154. Insome aspects, fins 159 may be formed to at or about 4 millimeters (mm)above the fluid channel 154 and at 16-40 fins per inch (fpi) across thefluid channel 154. Other extended surfaces, such as rounded or sharpridges, may also be used as the heat transfer surface 148.

In this example embodiment, the transport member 136 forms a top surface149 of the fluid pathways 160, thereby capping the fluid pathways 160 sothat the working fluid vapor phase 156 is constrained within the fluidpathways 160 until the vapor phase 156 flows into the troughs 158. Inalternative implementations, a separate member (e.g., a flat sheet orother member) may be used to vertically enclose or cap the fluidpathways 160 at a top of the pathways 160.

In this example implementation shown in FIGS. 3A-3C, there are two vaporconduits 146 with inlets 143, shown on opposed sides of the transportmember 136. In this example, each vapor conduit 146 is substantially“L”-shape with an extended portion towards the middle of the transportmember 136 and a stub portion that is positioned over a trough 158 ofthe evaporator 132.

FIG. 3C illustrates a top cross-section view of the evaporator 132. Asillustrated, the fluid channel 154 extends through the evaporator 132and, in this implementation, is transverse to the fluid pathways 160that are formed between the fins 159. As shown in dotted line here, theoutlet 150 of the transport member 136 extends parallel to the fluidchannel 154 (and transverse to the fluid pathways 160) and through acenter of the channel 154. The troughs 158 are also illustrated bydotted lines extending parallel to the outlet 150 and on opposed sidesof the fluid channel 154.

Turning briefly to FIG. 5B, a schematic isometric view of a condenserend 182 of the transport member 136 is illustrated. In some exampleimplementations, the condenser end 182 may correspond with theevaporator end 142 of the transport member 136, in that two outlets 145of the vapor conduits 146 are shown exiting the condenser end 182, witha single liquid conduit 144 having a liquid entrance 147 in thecondenser end 182. As illustrated in FIG. 5B, the condenser end 182includes an extended portion 184, which includes the outlets 145 of thevapor conduits 146 and a portion of the inlet 147 of the liquid conduit144. The condenser end 182 also includes a recessed portion 186 thatincludes a portion of the entrance 147 of the liquid conduit 144. Therecessed portion 186, as a solid portion of the transport member 136,extends length-wise through the transport member 136 to force a flow ofworking fluid vapor to circulate (e.g., naturally) through the extendedportions of the L-shaped vapor conduits 146 that extend the entirelength of the transport member 136 (and exit as shown in FIG. 5B at thecondenser end 182).

In some aspects, the inlets and outlets of the liquid conduit 144 or thevapor conduit 164, or both, may be adjustable (e.g., as manufactured) sothat a pressure loss of the working fluid that circulates through thetransport member 136, the evaporator 132, and the condenser 134 may bemore particularly matched to a required thermal load (e.g., to match aheat output of the heat generating electronic devices 124. For example,adjusting the inlets, the outlets, or both, may adjust a total pressureloss of the working fluid as it circulates through the thermosiphonsystem 130, which may adjust a cooling capacity of the working fluid.

In operation, a liquid phase 152 of a working fluid of the thermosiphoncooling system flows (e.g., naturally) through the liquid conduit 144 inthe transport member 136 towards the evaporator 132. As the liquid phase152 reaches the evaporator 132 positioned below the transport member136, the liquid phase 152 flows through an outlet 150 of the liquidconduit 144 and into the fluid channel 154. As the heat transfer surface148 is divided into the fluid pathways 160 (e.g., by the fins 159), theliquid phase 152 flows into the fluid pathways 160 from the outlet 150.

In the fluid pathways 160, the liquid phase 152 receives heat from oneor more heat generating devices (not shown) thermally coupled to theevaporator 132. As the heat is received, the liquid phase 152 begins toboil, e.g., change phase from liquid to a mixed-phase or vapor phase ofthe working fluid. As described previously, as the liquid phase 152changes to a vapor phase (or mixed phase) 156 in the fluid pathways 160,a density of the working fluid decreases, thereby increasing a flowvelocity of the working fluid in the fluid pathways 160 (which in turnincreases heat transfer to the working fluid in the fluid pathways 160).Further, in some example implementations, the fluid pathways 160 may bepitched away from the outlet 150 and towards the troughs 158.

The working fluid that is vertically enclosed within the fluid pathways160 by the top surface 149 and flows from the fluid pathways 160 intothe troughs 158 is the vapor or mixed phase 156 (e.g., all orsubstantially all). In some examples, the thermosiphon system may bedesigned such that most of the liquid phase 152 is vaporized within thefluid channels 160, without running the thermosiphon “dry” (e.g.,vaporizing all of the liquid phase 152) when the heat generating devicesare operating at peak or nameplate heat output (e.g., peak or nameplatepower, peak speed, or otherwise). As the vapor or mixed phase 156collects in the troughs 158, natural circulation (e.g., density and/orpressure differences) causes this phase 156 to migrate to the vaporconduits 146, and to circulate back to the condenser end 182 of thethermosiphon system in the transport member 136.

During operation, in some example aspects of the transport member 136,heat may be transferred from the vapor (or mixed phase) 156 to theliquid phase 152 as the two phases flow through the transport member 136(in opposite directions). For example, the transport member 136 mayinclude a heat transfer interface 188 that extends a length of thetransport member 136 between the liquid conduit 144 and the vaporconduits 146. The heat transfer interface 188 may be made of the same orsimilar material as the transport member 136 (e.g., copper, aluminum, orotherwise), or may be made of a material of higher thermal conductivitythan the transport member 136. In any event, as the vapor (or mixed)phase 156 circulates (e.g., naturally) toward the condenser, heat may betransferred from the phase 156 to the liquid phase 152. By “pre-heating”the liquid phase 152 (e.g., heating the phase 152 prior to entering thefluid pathways 160), additional vaporization of the liquid phase 152 inthe pathways 160 may be achieved.

FIGS. 4A-4C illustrate schematic cross-sectional side, isometric, andtop views, respectively, of an evaporator end of another exampleimplementation of a thermosiphon cooling system, such as thethermosiphon cooling system 130 shown in FIGS. 2A-2B. As illustrated inthe cross-section of FIG. 4A, an evaporator end 162 of the transportmember 136 is mounted on the evaporator 132. In this implementation ofthe transport member 136, the member 136 includes a single liquidconduit 164 and a single vapor conduit 166. As shown, the liquid conduit164 extends (e.g., in an “L” shaped cross-section) through a right sideportion (in these views) of the transport member 136 and above a heattransfer surface 168 of the evaporator 132. The heat transfer surface168, in this implementation, is positioned over a fluid channel 174 ofthe evaporator 132 that is parallel (e.g., exactly or substantially) tothe liquid conduit 164. Also, the heat transfer surface 168, in thisexample, includes multiple fins 161 that form fluid pathways 180 acrossthe evaporator 132 (e.g., transverse to the liquid conduit 164) withinthe fluid channel 174. In some aspects, fins 161 may be formed to at orabout 4 mm above the fluid channel 174 and at 16-40 fins per inch (fpi)across the fluid channel 174. Other extended surfaces, such as roundedor sharp ridges, may also be used as the heat transfer surface 168.

In this example embodiment, the transport member 136 forms a top surface169 of the fluid pathways 180, thereby capping the fluid pathways 180 sothat the working fluid vapor phase 176 is constrained within the fluidpathways 180 until the vapor phase 176 flows into the trough 178. Inalternative implementations, a separate member (e.g., a flat sheet orother member) may be used to vertically enclose or cap the fluidpathways 180 at a top of the pathways 180.

In this example implementation shown in FIGS. 4A-4C, there is one vaporconduit 166, shown on an opposed left side (in these views) of thetransport member 136 across from the liquid conduit 164. In thisexample, the vapor conduit 166 is an inverted “L”-shape with an extendedportion towards the middle of the transport member 136 and a stubportion that is positioned over a trough 178 of the evaporator 132.

FIG. 4C illustrates a top cross-section view of the evaporator 132. Asillustrated, the fluid channel 174 extends through the evaporator 132and, in this implementation, is also transverse to the fluid pathways180 that are formed between the fins 161. As shown in dotted line here,the inlet trough 177 extends parallel to the fluid channel 174 (andtransverse to the fluid pathways 180) and along a side of the channel174. The outlet trough 178 is also illustrated by a dotted lineextending parallel to the inlet trough 177 and on an opposed side of thefluid channel 174 relative to the inlet trough 177.

Turning briefly to FIG. 5C, a schematic isometric view of a condenserend 192 of the transport member 136 is illustrated. In some exampleimplementations, the condenser end 192 may correspond with theevaporator end 162 of the transport member 136, in that one vaporconduit 166 is shown exiting the condenser end 192, with a single liquidconduit 164 having a liquid entrance in the condenser end 192. Asillustrated in FIG. 5C, the condenser end 192 includes an extendedportion 194, which includes the exit of the vapor conduit 166 and aportion of the entrance of the liquid conduit 164. The condenser end 192also includes a recessed portion 196 that includes a portion of theentrance of the liquid conduit 164. The recessed portion 196, as a solidportion of the transport member 136 (underneath the vapor conduit 166),extends length-wise through the transport member 136 to force a flow ofworking fluid vapor to circulate (e.g., naturally) through the extendedportion of the inverted L-shaped vapor conduit 166 that extends theentire length of the transport member 136 (and exits as shown in FIG. 5Cat the condenser end 192).

In operation, a liquid phase 172 of a working fluid of the thermosiphoncooling system flows (e.g., naturally) through the liquid conduit 164 inthe transport member 136 towards the evaporator 132. As the liquid phase172 reaches the evaporator 132 positioned below the transport member136, the liquid phase 172 flows through an outlet 170 of the liquidconduit 164, into an inlet trough 177, and into the fluid channel 174.As the heat transfer surface 168 is divided into the fluid pathways 180(e.g., by the fins 161), the liquid phase 172 flows into the fluidpathways 180 from the outlet 170.

In the fluid pathways 180, the liquid phase 172 receives heat from oneor more heat generating devices (not shown) thermally coupled to theevaporator 132. As the heat is received, the liquid phase 172 begins toboil, e.g., change phase from liquid to a mixed-phase or vapor phase ofthe working fluid. As described previously, as the liquid phase 172changes to a vapor phase (or mixed phase) 176 in the fluid pathways 180,a density of the working fluid decreases, thereby increasing a flowvelocity of the working fluid in the fluid pathways 180 (which in turnincreases heat transfer to the working fluid in the fluid pathways 180).Further, in some example implementations, the fluid pathways 180 may bepitched away from the inlet trough 177 and towards the outlet trough178.

The working fluid that flows from the fluid pathways 180 into the outlettrough 178 is the vapor or mixed phase 176 (e.g., all or substantiallyall). In some examples, the thermosiphon system may be designed suchthat most of the liquid phase 172 is vaporized within the fluid channels180, without running the thermosiphon “dry” (e.g., vaporizing all of theliquid phase 172) when the heat generating devices are operating at peakor nameplate heat output (e.g., peak or nameplate power, peak speed, orotherwise). As the vapor or mixed phase 176 collects in the outlettrough 178, natural circulation (e.g., density and/or pressuredifferences) cause this phase 176 to migrate to the vapor conduit 166,and to circulate back to the condenser end 192 of the thermosiphonsystem in the transport member 136.

During operation, in some example aspects of the transport member 136,heat may be transferred from the vapor (or mixed phase) 176 to theliquid phase 172 as the two phases flow through the transport member 136(in opposite directions). For example, the transport member 136 mayinclude a heat transfer interface 198 that extends a length of thetransport member 136 between the liquid conduit 164 and the vaporconduit 166. The heat transfer interface 198 may be made of the same orsimilar material as the transport member 136 (e.g., copper, aluminum, orotherwise), or may be made of a material of higher thermal conductivitythan the transport member 136. In any event, as the vapor (or mixed)phase 176 circulates (e.g., naturally) toward the condenser, heat may betransferred from the phase 176 to the liquid phase 172. By “pre-heating”the liquid phase 172 (e.g., heating the phase 172 prior to entering thefluid pathways 180), additional vaporization of the liquid phase 172 inthe pathways 180 may be achieved.

FIGS. 5A illustrates a schematic side view of a condenser end of exampleimplementations of a thermosiphon cooling system. FIG. 5A, as shown,illustrates a side view of either of condenser ends 182 or 192, as shownin FIGS. 5B or 5C, respectively. This side view illustrates condenserends 182 or 192 positioned within the condenser 134. Generally, thevapor (or mixed) phase of a working fluid that circulates out of thecondenser ends 182/192 enters the condenser 134, is cooled, andcondenser to a liquid phase of the working fluid. As shown, extendedportions 184 and 194 of the condenser ends 182 and 192 provide outletsfor the vapor (or mixed) phase 156 and 176.

Further, the recessed portions 186 and 196 of the condenser ends 182 and192 provide at least part of inlets for the liquid phase 152 and 172. Insome implementations, the inlets are offset in the condenser ends 182and 192 so that liquid working fluid is directed (e.g., by gravity)towards the inlets.

In alternative embodiments, the fluid pathways 160 may be orientedparallel (e.g., exactly or substantially) to the fluid channel 154.Thus, in aspects that include the fins 159, the fins 159 (or ridges, forexample) are oriented to form the fluid pathways 160 parallel to thefluid channel 154.

FIGS. 6A-6B illustrate schematic side and top cross-sectional views,respectively of an evaporator of another example implementation of athermosiphon cooling system. For example, FIGS. 6A-6B show an evaporator132 in which fluid pathways extend through the evaporator parallel witha fluid channel within the evaporator. Turning to FIG. 6A, The transportmember 136 is illustrated on top of the evaporator 132 and forming a topsurface 205 for fluid pathways 220. The fluid pathways 220 are formedbetween heat transfer surfaces 210 (e.g., fins or ridges) through afluid channel 215 of the evaporator 132. FIG. 6B illustrates a topcross-section of the evaporator 132 shown in FIG. 6A. As illustrated,the heat transfer surfaces 210 extend through, and parallel with, thefluid channel 215 (relative to the longest dimension of the channel215). The fluid pathways 220 also are formed to extend parallel with thelongest dimension of the fluid channel 215.

In operation, liquid working fluid may flow into an inlet area 225 ofthe evaporator 132, and, due to heat transfer from one or more heatgenerating electronic devices thermally coupled with the evaporator, mayphase change to a vapor or mixed-phase working fluid through the fluidpathways 220. As the fluid pathways 220 are enclosed by the top surface205, the working fluid is constrained to flow through the fluid pathways220 to an outlet area 230. The vapor or mixed-phase working fluid exitsthe evaporator 132, and into a vapor conduit of the transport member136, from the outlet area 230.

FIG. 6C illustrates a top cross-section of another implementation of theevaporator 132 shown in FIG. 6A, which includes one or more spacers 235.In this implementation, the spacers 235 may be solid or mostly solidmembers that are inserted into the fluid channel 215 of the evaporator132. The spacers 235 may be substantially the same depth of the fluidchannel 215 and vertically extend up to the top surface 205. In someimplementations, the spacers 235 may effectively decrease a heattransfer volume of the evaporator 132 (e.g., by decreasing a number ortotal volume of fluid pathways 220), thereby decreasing a total coolingpower of the thermosiphon. Spacers 235, although illustrated in FIG. 6Cas extending along sides of the fluid channel 215, may be any shape orsize and may be tailored to effectively decrease any desired heattransfer volume. For example, spacers 235 may restrict the flow andbalance a liquid flow rate of the working fluid through the evaporator132. As noted, a thickness and size of the spacers 235 (or a singlespacer 235) can be changed to adjust the flow rate based on the expectedheat load. The spacer 235 or spacers 235 may also reduce or eliminate aneed to have different cut out sizes for the liquid or vaporinlet/outlet in the transport member 126. As shown, spacers 235 areadded on the two sides of the fins 210 to eliminate bypass flow aroundthe fins 210 and guide all of the working fluid through the fluidpathways 220 between the fins 210. Thus, thermal performance may betailored for a cooling power of the thermosiphon to a particularapplication (e.g., a particular heat output of one or more heatgenerating electronic devices).

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of what is described. Further, in someimplementations, a phase change material may be positioned, for example,between an evaporator of a thermosiphon and one or more heat generatingelectronic devices to increase a thermal contact area between theevaporator and the devices. As another example, in some implementationsa heat transfer surface of an evaporator of a thermosiphon may notinclude fins or ridges. Accordingly, other embodiments are within thescope of the following claims.

What is claimed is:
 1. A thermosiphon, comprising: a condenser; an evaporator that comprises a fluid channel and a heat transfer surface, the heat transfer surface defining a plurality of fluid pathways in the fluid channel that extend through the fluid channel, the evaporator configured to thermally couple to one or more heat-generating electronic devices; and a transport member that fluidly couples the condenser and the evaporator, the transport member comprising a liquid conduit that extends through the transport member to deliver a liquid phase of a working fluid into the fluid pathways, the transport member further comprising a surface to vertically enclose the plurality of fluid pathways.
 2. The thermosiphon of claim 1, wherein the fluid channel is oriented transverse to the fluid pathways.
 3. The thermosiphon of claim 1, wherein the transport member further comprises a vapor conduit that extends through the transport member to receive a mixed-phase of the working fluid from the fluid pathways.
 4. The thermosiphon of claim 1, wherein the fluid pathways are configured to transfer heat from the heat-generating electronic devices to the working fluid to change the working fluid from the liquid phase to the mixed-phase between inlets of the fluid pathways and outlets of the fluid pathways.
 5. The thermosiphon of claim 4, wherein the inlets of the fluid pathways are positioned at a first end of the fluid pathways and the outlets of the fluid pathways are positioned at a second end of the fluid pathways opposite the first end.
 6. The thermosiphon of claim 4, wherein the inlets of the fluid pathways are positioned at a midpoint of the fluid pathways, and the outlets of the fluid pathways are positioned at opposed ends of the fluid pathways.
 7. The thermosiphon of claim 1, wherein the heat transfer surface comprises a plurality of fins or ridges, and the plurality of fins or ridges form the plurality of fluid pathways in the fluid channel, and the plurality of fluid pathways extend transverse to the fluid channel.
 8. The thermosiphon of claim 1, wherein the heat transfer surface comprises a plurality of fins or ridges, and the plurality of fins or ridges form the plurality of fluid pathways in the fluid channel, and the plurality of fluid pathways extend parallel with the fluid channel.
 9. The thermosiphon of claim 1, wherein the transport member further comprises a heat transfer interface between the liquid conduit and the vapor conduit to transfer heat from the working fluid in the vapor conduit to the working fluid in the liquid conduit.
 10. The thermosiphon of claim 1, wherein the liquid phase is at or near a saturation temperature of the working fluid.
 11. The thermosiphon of claim 3, wherein the vapor conduit comprises at least two vapor conduits.
 12. The thermosiphon of claim 3, wherein at least a portion of the vapor conduit is positioned in an upper half of the transport member, and at least a portion of the liquid conduit is positioned in a lower half of the transport member.
 13. The thermosiphon of claim 3, wherein the transport member comprises a condenser end that comprises an inlet of the liquid conduit that is offset in the transport member relative to an outlet of the vapor conduit in the condenser end of the transport member.
 14. The thermosiphon of claim 3, wherein a cross sectional area of the liquid conduit and a cross-sectional area of the vapor conduit is based, at least in part, on a heat load of the heat-generating electronic devices.
 15. The thermosiphon of claim 3, wherein the transport member slopes from the condenser to the evaporator, and a magnitude of the slope defines, at least in part, a liquid head of the working fluid.
 16. The thermosiphon of claim 15, wherein the liquid head of the working fluid is equal to a sum of a plurality of pressure losses in a closed loop fluid circuit that comprises the liquid conduit, the evaporator, the vapor conduit, and the condenser.
 17. The thermosiphon of claim 1, further comprising a spacer positioned in the fluid channel.
 18. A method for cooling heat generating electronic devices in a data center, comprising: flowing a liquid phase of a working fluid from a condenser of a thermosiphon to an evaporator of the thermosiphon in a transport member of the thermosiphon; flowing the liquid phase of the working fluid into a fluid channel of the evaporator from the transport member; flowing the liquid phase of the working fluid from the fluid channel to a plurality of fluid pathways that extend through the fluid channel and are enclosed between the evaporator and the transport member; boiling at least a portion of the liquid working fluid flowing in the plurality of fluid pathways based on a transfer of heat from at least one data center heat generating device that is thermally coupled to the evaporator; and flowing a mixed phase of the working fluid out of the plurality of fluid pathways into at least one vapor conduit of the transport member and to the condenser.
 19. The method of claim 18, further comprising: transferring heat from the heat-generating electronic devices to the working fluid through a heat transfer surface that forms the plurality of fluid pathways; and changing the working fluid from the liquid phase to the mixed-phase between inlets of the fluid pathways and outlets of the fluid pathways.
 20. The method of claim 19, further comprising: flowing the liquid phase into the inlets of the fluid pathways positioned at a first end of the fluid pathways; and flowing the mixed phase out of the outlets of the fluid pathways positioned at a second end of the fluid pathways opposite the first end.
 21. The method of claim 19, further comprising: flowing the liquid phase into the inlets of the fluid pathways positioned at a midpoint of the fluid pathways; and flowing the mixed phase out of the outlets of the fluid pathways positioned at opposed ends of the fluid pathways.
 22. The method of claim 19, further comprising flowing the working fluid through a plurality of fins or ridges positioned that form the plurality of fluid pathways in the fluid channel.
 23. The method of claim 22, wherein flowing the working fluid through a plurality of fins or ridges that form the plurality of fluid pathways in the fluid channel comprises flowing the working fluid transversely from the fluid channel through the plurality of fluid pathways.
 24. The method of claim 18, further comprising transferring heat from the working fluid in the vapor conduit to the working fluid in the liquid conduit through a heat transfer interface between the liquid conduit and the vapor conduit in the transport member.
 25. The method of claim 18, wherein the liquid phase is at or near a saturation temperature of the working fluid.
 26. The method of claim 18, further comprising flowing the mixed phase of the working fluid through the at least one vapor conduit to the condenser.
 27. A data center cooling system, comprising: a tray sub-assembly configured to engage with a rack; a support board mounted on the tray sub-assembly, the support board comprising a heat-generating computing device; and a thermosiphon system, comprising: a condenser; a flow boiling evaporator that comprises at least one fluid pathway configured to receive a flow of a working fluid in liquid phase and output the flow of the working fluid in a mixed liquid-vapor phase based on heat transferred from the heat-generating computing device to the flow of the working fluid; and a transport tubular that fluidly couples the condenser and the evaporator.
 28. The data center cooling system of claim 27, wherein the evaporator comprises a fluid channel and a plurality of fluid pathways that extend through the fluid channel, and the transport tubular comprises: a liquid carrier that extends through the transport tubular and is oriented transverse to the fluid pathways to deliver the flow of the working fluid in liquid phase into the fluid pathways; and a vapor carrier that extends through the transport tubular to receive the flow of the working fluid in mixed liquid-vapor phase from the fluid pathways.
 29. The data center cooling system of claim 28, wherein the plurality of fluid pathways are defined between a plurality of heat transfer surfaces.
 30. The data center cooling system of claim 28, wherein inlets of the plurality fluid pathways are positioned at a first end of the fluid pathways and outlets of the plurality of fluid pathways are positioned at a second end of the fluid pathways opposite the first end.
 31. The data center cooling system of claim 28, wherein inlets of the plurality of fluid pathways are positioned near or at a midpoint of the fluid pathways, and outlets of the plurality of fluid pathways are positioned at opposed ends of the fluid pathways. 